Do I need testing before targeted therapy?

Usually yes. Most of the time your tumor has to be tested first, to prove it carries the specific target the drug is built to hit. Without that proof the drug has nothing to attack.

The test is called biomarker testing. It is also called tumor testing, tumor profiling, molecular profiling, or somatic testing. All those names mean the same thing: a lab examines your cancer cells for genes, proteins, and other markers that a treatment could act on.

What the lab is actually looking for

Targeted drugs work on proteins that control how cancer cells grow, divide, and spread. Each drug is designed for one target. So the question is narrow. Does your cancer carry that target or not?

The report you get back lists the biomarkers found in your cancer and any treatments that might match. There are three useful outcomes. A match may point to a drug approved for your cancer. A match may point to a drug approved for a different cancer that carries the same marker, which would be off-label use. Or the test may find a marker that predicts a certain drug will fail, which saves you from a treatment that was never going to help.

For some drugs the test is not optional in any sense. The FDA label for trastuzumab, a targeted antibody for HER2-positive breast and stomach cancer, instructs doctors to select patients using an FDA-approved companion diagnostic. A companion diagnostic is a test approved alongside the drug to identify who should get it.

When testing is not the gatekeeper

There are exceptions. In chronic myelogenous leukemia, nearly everyone with that disease has the target, so the drug can be given without hunting for it in each person.

Hormone therapy is another case where the test is routine and simple rather than elaborate. Breast tumors are checked for estrogen and progesterone receptors as a standard part of the pathology work, not as a separate expedition.

Getting the sample, and what can go wrong

You may need a biopsy, a procedure in which a doctor removes a piece of the tumor. Biopsies carry risk, and the risk depends on how big the tumor is and where it sits. Your doctor should explain the specific risks for your tumor before you agree.

If you already had surgery, stored tissue from that operation can often be used, so ask before agreeing to a fresh biopsy. For blood cancers, or when a tumor cannot be reached safely with a needle, a blood test called a liquid biopsy is an option. Two liquid biopsy tests have FDA approval: Guardant360 CDx and FoundationOne Liquid CDx.

Several things can derail this. There may not be enough tumor tissue in the sample to run the test. The lab may find a match to a drug that your insurer will not pay for off label. And even a clean match is not a guarantee, because other features of your cancer or how your body handles the drug can still block it.

Biomarker testing is also not available at every hospital. Ask whether your center runs it in house or ships samples out, and ask how long results take, because that wait can shape when treatment starts.

Cost, and one thing worth doing

Prices vary widely by test, cancer type, and plan. For people with advanced cancer, Medicare and Medicaid cover some biomarker tests. Private insurers often cover a test when there is solid evidence it is needed to guide treatment, and may refuse tests they consider experimental. Clinical trials that include biomarker testing sometimes cover the cost, so ask the study coordinator.

One practical step: ask for a copy of your biomarker report and keep it. If your cancer returns, the markers may have changed and your doctor may retest, and having the earlier report makes the comparison possible.

Sources

https://www.cancer.gov/about-cancer/treatment/types/targeted-therapies

https://www.cancer.gov/about-cancer/treatment/types/biomarker-testing-cancer-treatment

https://www.accessdata.fda.gov/drugsatfda_docs/label/2018/103792s5345lbl.pdf